INTERNAL ANATOMY OF INSECTS. yi. 
diverging embrace the @sophagus, and dipping below it 
and the intestines,—a situation they maintain to the end 
of their course,—and in their further progress uniting at 
intervals and dilating into several knots or ganglions, 
compose their spinal marrow. ‘This part is so named, 
from a supposed analogy to the spinal marrow of verte- 
brate animals, which however admits of some degree of 
doubt; yet, since it mixes the functions of that organ 
with those of the great sympathetic nerves, the denomi- 
nation is not wholly improper, and may be retained. 
Though this chord is usually double when it first pro- 
ceeds from the brain, and surrounds the a@sophagus like 
a collar, yet in some insects it may be called a single 
chord. This is the case with that of the common louse, 
in which Swammerdam could perceive no opening for 
the transmission of the part just named*; if he was not 
mistaken in this, the brain, as well as the rest of the spi- 
nal marrow in that animal, would be below the intestines ; 
from the figures of Treviranus it should seem that the 
spiders, at least Clubiona atrox, are similarly circum- 
stanced>; in the cheese-maggot, which turns to a two- 
winged fly (Tyrophaga putris K.), the chord is also sin- 
gle, but it has a small orifice through which the gullet 
passes*. At the union of the chords in other cases be- 
low that organ, a knot or ganglion is usually formed, and 
an alternate succession of internodes and ganglions com- 
monly follows to the end. The internodes also may ge- 
nerally be stated to consist of a double chord, though 
in many cases the two chords unite and become one, or 
@ Prats XXI. Fic. 8. Swamm. Bibl. Nat. i. 36. b. 
» Arachnid. t.v. f. 49. © Swamm. wdi supr. t. xii. f. 7. 
